The CollabVM Advent Calendar is an advent calendar belonging to the CollabVM project, which is held every December (not always, however). The advent calendar has been held yearly since 2018, and ongoing. Originally, up until 2021's advent calendar, a new OS (usually an obscure, and interesting one) would get posted every day until the 25th of December. Now it's a Crusty Windows bootleg ever since it launched in late 2021. In fact, the very last entry of the 2021 Advent Calendar was a Crusty Windows bootleg, by the name of Windows 7 Christmas Edition 2015 created by The Rain, which released to TeamOS.